A distributed optical fiber sensor is described in an article by A. Masoudi et al. titled “A distributed optical fibre dynamic strain sensor based on phase-OTDR” and published in Measurement Science and Technology 24 (2013) 085204. Such a fiber sensor may be used in an intrusion sensor with a buried optical fiber. The vibration exerted on the fiber when an intruder walks over the ground above the fiber influences optical propagation in the fiber. It results in local strain (stretching of the fiber). It is possible to detect locations at which this effect occurs in fibers of more than ten kilometer long.
Masoudi et al. describe a measurement system wherein a light pulse from a laser source is fed into the optical fiber. Backscattered light from the fiber is fed through a Mach Zehnder interferometer and a 3×3 optical coupler to three detectors. In the Mach Zehnder interferometer the backscattered light is split and the split light is fed in parallel thorough two fiber branches of different optical length. The 3×3 optical coupler combines the light from the fiber branches with three different relative phase offsets. This makes it possible to compute phase measurements from the detected signals independent of fading effects in the backscattered signal.
EP2792996 also discloses a Mach Zehnder interferometer coupled to 3×3 optical coupler, be it for a different measurement
Masoudi et al. disclose that for each respective time point as a function of time from transmission of the pulse, changes in the measured phase represents a phase change induced between Rayleigh scattering from a pair of points of the sensing fiber at a respective distance from the input side of the fiber. The phase change is directly related to the change in strain between the two points.
Masoudi et al. disclose measurement obtained when the fiber is subjected locally to temporally periodic strain variations at two positions along the fiber. A discrete Fourier transform was applied to measured phase values for successive pulses at the same time from transmission of the pulses. This yielded a plot of the Fourier transform phase as a function of the Fourier transform frequency and the time from transmission. This plot showed peaks at the Fourier transform frequency corresponding to the imposed vibration frequencies and the time from transmission corresponding to the distance to the location where the vibration was imposed.
Rayleigh backscattering has a low amplitude. On one hand this enables detection along a very long optical fiber. But on the other hand it necessitates Masoudi to use of optical amplifiers to amplify the emitted and backscattered light.
The spatial resolution of this measurement depends on the length of the pulse. Shorter pulses give better resolution. On the other hand, shorter pulses mean lower detectable optical energy and also affect the performance of certain phase detection technologies. Increasing the transmitted pulse intensity can lead to other problems, such as saturation of optical amplifiers used in the detection system and disturbance of the measurements of Rayleigh scattering due to inelastic scattering. As reported by S. P. Singh and N. Singh, in “Nonlinear effects in optical fibers: origin, management and applications” (Progress In Electromagnetics Research, PIER 73, 249-275, 2007) optical fibers produce inelastic scattering.
US2007165238 discloses an interferometric sensing arrangement includes with four lasers operating at different fixed wavelengths separated by a few nanometer. An optical coupler couples the multiple wavelength light to the interferometric sensor. The interferometric sensor uses interference between reflections from a fiber end face and a discrete reflector in the fiber. The different pulsed laser sources are used successively in time division multiplexing to measure the response at the different wavelengths. Difference ratios between measured intensities at different wavelengths are used as compensation for losses, e.g. due to fiber bending. Optical path lengths are determined based on measured amplitude responses.
US2007171402 discloses an optical sensing system that uses light scattered from a sensing fiber to sense conditions along the fiber. A receiver obtains a frequency of a Brillouin component of the received scattered light, to deduce the conditions.
EP0983486 discloses a system for measuring the strain in a structure using an optic fiber is incorporated in the structure like a building. The fiber contains a discrete interferometer positioned at a point where the strain is to be measured. A plurality of discrete sensing interferometers may be positioned along a length of optical fiber, each by means of pairs of reflectors created by prior local modification the refractive index of in the fiber. Path length changes in the interferometers are measured. Loss and temperature distributions are determined by detecting the Raman scattering spectrum.